PROJECT SUMMARY/ABSTRACT Mucolipidosis type IV (MLIV) is a lysosomal storage disorder that mainly affects the brain, eyes, and stomach. The disease is caused by a loss of function mutation in the TRPML1 ion channel. We discovered abnormally high zinc levels in lysosomes of MLIV patient fibroblasts and cell culture model of MLIV. Moreover, we found elevated zinc levels in cortical brain tissues taken from the mucolipin-1 knockout (Mcoln1?/? KO) MLIV mouse model. In an attempt to explain zinc imbalance in MLIV, we identified a TRPML1 interaction partner called Transmembrane (TMEM)-163 that happens to bind and transport zinc. The main goal of the previous R15 grant is to characterize the role of TMEM163 and its relevance to MLIV. The data arising from the project show for the first time that TMEM163 is a zinc efflux transporter. Interestingly, we also found that TMEM163 interacts with ZNT4, a zinc transporter implicated by another lab as responsible for lysosomal zinc accumulation upon knockdown of TRPML1 in cultured cells. Zinc flux assays of the TMEM163-ZNT4 heterodimer slightly differed from homodimeric TMEM163, suggesting that the interaction may modulate TMEM163's function. Meanwhile, RNA sequencing (RNA-seq) of cortical brain tissues from Mcoln1?/? KO and wild-type littermate mice revealed that mouse Tmem163 and several ZnT (Slc30) zinc transporters transcripts are prominently down-regulated. Thus, we hypothesize that TMEM163 and specific zinc transporters may potentially contribute to MLIV pathology. The main goal of this R15 renewal is to further dissect the roles of TMEM163 and other zinc transporters in MLIV disease. The first aim will investigate the interaction and functional difference between TMEM163 and ZNT2 (lysosomal zinc) transporter, and TMEM163 and ZNT3 (vesicular zinc) transporter based on the fact that these related proteins have similar function as ZNT4 and localize within membrane compartments. Secondly, we aim to validate our initial findings of differentially expressed Tmem163 and ZnT (Slc30) zinc transporter genes from Mcoln1?/? brain tissues. Validation of the RNA-seq data could help explain the mechanistic processes underlying zinc dyshomeostasis in MLIV cells. Finally, we aim to screen and identify drugs that will augment or inhibit the efflux activity of TMEM163 for use in functional assays or potential therapeutics. Ultimately, the data that will be gathered from this proposal would fill current gaps in our knowledge on the physiological and pathological roles contributed by TMEM163 and other zinc transporters in the etiology and disease progression underlying intracellular or lysosomal zinc accumulation MLIV and other neurodegenerative diseases.